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1.1.5 Classification of Errors

Implementation Requirements

The language definition classifies errors into several different categories: 
Errors that are required to be detected prior to run time by every Ada implementation;
These errors correspond to any violation of a rule given in this International Standard, other than those listed below. In particular, violation of any rule that uses the terms shall, allowed, permitted, legal, or illegal belongs to this category. Any program that contains such an error is not a legal Ada program; on the other hand, the fact that a program is legal does not mean, per se, that the program is free from other forms of error.
The rules are further classified as either compile time rules, or post compilation rules, depending on whether a violation has to be detected at the time a compilation unit is submitted to the compiler, or may be postponed until the time a compilation unit is incorporated into a partition of a program. 
Ramification: See, for example, 10.1.3, “Subunits of Compilation Units”, for some errors that are detected only after compilation. Implementations are allowed, but not required, to detect post compilation rules at compile time when possible. 
Errors that are required to be detected at run time by the execution of an Ada program;
The corresponding error situations are associated with the names of the predefined exceptions. Every Ada compiler is required to generate code that raises the corresponding exception if such an error situation arises during program execution. [If such an error situation is certain to arise in every execution of a construct, then an implementation is allowed (although not required) to report this fact at compilation time.]
Bounded errors;
The language rules define certain kinds of errors that need not be detected either prior to or during run time, but if not detected, the range of possible effects shall be bounded. The errors of this category are called bounded errors. The possible effects of a given bounded error are specified for each such error, but in any case one possible effect of a bounded error is the raising of the exception Program_Error.
Erroneous execution.
In addition to bounded errors, the language rules define certain kinds of errors as leading to erroneous execution. Like bounded errors, the implementation need not detect such errors either prior to or during run time. Unlike bounded errors, there is no language-specified bound on the possible effect of erroneous execution; the effect is in general not predictable.
Ramification: Executions are erroneous, not programs or parts of programs. Once something erroneous happens, the execution of the entire program is erroneous from that point on, and potentially before given possible reorderings permitted by 11.6 and elsewhere. We cannot limit it to just one partition, since partitions are not required to live in separate address spaces. (But implementations are encouraged to limit it as much as possible.)
Suppose a program contains a pair of things that will be executed “in an arbitrary order.” It is possible that one order will result in something sensible, whereas the other order will result in erroneous execution. If the implementation happens to choose the first order, then the execution is not erroneous. This may seem odd, but it is not harmful.
Saying that something is erroneous is semantically equivalent to saying that the behavior is unspecified. However, “erroneous” has a slightly more disapproving flavor. 

Implementation Permissions

[ An implementation may provide nonstandard modes of operation. Typically these modes would be selected by a pragma or by a command line switch when the compiler is invoked. When operating in a nonstandard mode, the implementation may reject compilation_units that do not conform to additional requirements associated with the mode, such as an excessive number of warnings or violation of coding style guidelines. Similarly, in a nonstandard mode, the implementation may apply special optimizations or alternative algorithms that are only meaningful for programs that satisfy certain criteria specified by the implementation. In any case, an implementation shall support a standard mode that conforms to the requirements of this International Standard; in particular, in the standard mode, all legal compilation_units shall be accepted.] 
Discussion: These permissions are designed to authorize explicitly the support for alternative modes. Of course, nothing we say can prevent them anyway, but this (redundant) paragraph is designed to indicate that such alternative modes are in some sense “approved” and even encouraged where they serve the specialized needs of a given user community, so long as the standard mode, designed to foster maximum portability, is always available. 

Implementation Advice

If an implementation detects a bounded error or erroneous execution, it should raise Program_Error.
Implementation Advice: If a bounded error or erroneous execution is detected, Program_Error should be raised.

Wording Changes from Ada 83

Some situations that are erroneous in Ada 83 are no longer errors at all. For example, depending on the parameter passing mechanism when unspecified is possibly nonportable, but not erroneous.
Other situations that are erroneous in Ada 83 are changed to be bounded errors. In particular, evaluating an uninitialized scalar variable is a bounded error. The possible results are to raise Program_Error (as always), or to produce a machine-representable value (which might not be in the subtype of the variable). Violating a Range_Check or Overflow_Check raises Constraint_Error, even if the value came from an uninitialized variable. This means that optimizers can no longer “assume” that all variables are initialized within their subtype's range. Violating a check that is suppressed remains erroneous.
The “incorrect order dependences” category of errors is removed. All such situations are simply considered potential nonportabilities. This category was removed due to the difficulty of defining what it means for two executions to have a “different effect.” For example, if a function with a side effect is called twice in a single expression, it is not in principle possible for the compiler to decide whether the correctness of the resulting program depends on the order of execution of the two function calls. A compile time warning might be appropriate, but raising of Program_Error at run time would not be. 

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